In order to read out data from a floppy disc and to store this data in a random access memory (RAM) used by a microcomputer (MPU), a buffer has been provided in the control of the floppy disc drive so that data which has been read out from that disc is transferred to the RAM after it first has been stored in that buffer. The MPU is made operative to perform data processing using that data or to rewrite a portion of the data and to store it again on the floppy disc through the buffer.
In previously proposed systems, the reading of data from the disc has been performed for respective regions of the disc, called "sectors". Therefore, the buffer has been provided with a capacity corresponding to one sector or an integral multiple thereof. In such systems, moreover, even when the MPU makes use of only a portion of the data in one sector of the floppy disc, the contents of the sector as a whole are read out to the buffer and are then wholly transferred to the RAM. Since the undesired data is also transferred with the desired data, the data bus is occupied for an unnecessary time in this data transfer.
Even when the MPU rewrites predetermined data of one sector, all the data of the sector is transferred from the buffer, and the MPU then accesses the RAM to rewrite the data. After that, the data for the whole sector, including the corrected data, is stored again on the disc through the buffer. This results in a useless process in which data requiring no rewriting is also transferred to the RAM.
In view of the facts thus far described, it is desired that the microcomputer system be constructed such that only the desired portion of the data in the buffer disposed in a peripheral storage control is transferred to the RAM at any given time. In case the data in the buffer is to be rewritten, moreover, it is desired that the buffer be accessed directly by the MPU so that the data may be rewritten. In order to construct the peripheral storage control of an integrated circuit in this instance, and also avoid any need for increasing the number of pins necessary for the external storage control, it is desirable that the foregoing objectives be realized by using the signal lines, which are provided in the conventional microcomputer, as they are as much as possible, i.e. without increasing the number of signal lines provided between major components of the system.